Pneumococcal surface protein A(PspA) is a virulence factor that slows the clearance of Streptococcus pneumoniae from the blood. PspA is, also, highly immunogenic and elicits antibodies that can protect against fatal infections with S. pneumoniae in mice. Although PspAs are serologically variable, antisera raised against individual PspAs are sufficiently cross-reactive, that they can frequently protect against infections with pneumococci possessing serologically distinguished PspAs. The complete sequence has been determine for pspA of pneumococcal strain Rx1. The N-terminal 49 percent of the inferred protein sequence is highly charged and consistent with an alpha- helix. This region of the molecule is serologically variable and contains the epitopes recognized by all known protective monoclonal antibodies (MAbs) to PspA. The alpha-helical domain of PspA is made up of three coiled-coil motifs of about 90 amino acids each separated by two short sequences expected to interrupt the coiled-coil conformation. Just C-terminal to the alpha-helical domain is a proline-rich domain followed by a large repeat domain. PCR analysis and Southern blots have shown more variability in the alpha-helical domain that in the C-terminal half of PspA. MAbs reactive with surface exposed Rx1 PspA recognize epitopes in the most N-terminal and most C-terminal of the three coiled-coil regions. The middle coiled-coil region appears, so far, to be immunologically silent. The epitopes of the C-terminal coiled-coil region are able to elicit protective antibodies and are much more cross- reactive with other PspAs than epitopes in the N-terminal coiled-coil region. The proposed studies will attempt to confirm and extend this finding by mapping additional protection-eliciting epitopes in Rx1 PspA and in other serologically different PspAs. The investigator plans to determine the structural basis for the variability of PspAs by sequencing serologically different PspAs (another complete pspA and 3-5 additional alpha-helical coding regions). For each of the 4-6 new alpha-helical sequences, he will also attempt to determine whether, like Rx1 PspA, the C-terminal third of each alpha-helical region contains the most highly cross-reactive protection- eliciting epitopes. In addition, the investigator plans to make a more complete map of protection-eliciting epitopes of Rx1 and possibly one of the new alpha-helical sequences. Together, these experiments will allow the investigator to identify the relationship between 1) conserved and variable regions of PspA; 2) protection-eliciting area of PspA; and 3) the structural domains revealed by the Rx1 pspA sequence. Finally, these data will allow Dr. McDaniel to identify those regions of PspA most appropriate for inclusion in a protein based vaccine to protect against infection with S. pneumoniae.